Text clustering

What is clustering?
Text Clustering 2
Inter-cluster
distances are
maximized
Intra-cluster
distances are
minimized
 Finding groups of objects such that the objects in a group will be similar (or
related) to one another and different from (or unrelated to) the objects in other
groups.
 Partition unlabeled examples into disjoint subsets of clusters, such that:
• Examples within a cluster are very similar
• Examples in different clusters are very different
 Discover new categories in an unsupervised manner (no sample category labels
provided).

• Grouping of text documents into meaningful clusters in an unsupervised
manner.
• Cluster Hypothesis : Relevant documents tend to be more similar to each
other than to non-relevant ones.
Text Clustering 3
Document Clustering
Government
Science
Arts

Document Clustering
Motivation
• Automatically group related documents based on their contents
• No predetermined training sets or taxonomies
• Generate a taxonomy at runtime
Clustering Process
• Data preprocessing:
– remove stop words, stem, feature extraction, lexical analysis, etc.
• Hierarchical clustering:
– compute similarities applying clustering algorithms.
• Model-Based clustering (Neural Network Approach):
– clusters are represented by “exemplars”. (e.g.: SOM)
Text Clustering 4

Document Clustering
Clustering of documents based on the similarity of their content improve the
search effectiveness in terms of:
1. Improving Search Recall
When a query matches a document its whole cluster can be returned.
2. Improving Search Precision
Grouping the documents into a much smaller number of groups of related
documents
3. Scatter/Gather
Enhance the efficiency of human browsing of a document collection when
a specific search query cannot be formulated.
4. Query-Specific Clustering
The most related documents will appear in the small tight clusters, nested
inside bigger clusters containing less similar documents.
Text Clustering 5

Text Clustering 6
Text Clustering
Overview
Standard (Text) Clustering Methods
 Bisecting k-means
 Agglomerative Hierarchical Clustering
Specialised Text Clustering Methods
 Suffix Tree Clustering
 Frequent-Termset-Based Clustering
Joint Cluster Analysis
 Attribute data: text content
 Relationship data: hyperlinks

Text Clustering 7
Text Clustering
Bisecting k-means [Steinbach, Karypis & Kumar 2000]
K-means
Bisecting k-means
 Partition the database into 2 clusters
 Repeat: partition the largest cluster into 2 clusters . . .
 Until k clusters have been discovered
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10

Text Clustering 8
Text Clustering
Bisecting k-means
Two types of clusterings
• Hierarchical clustering
• Flat clustering: any cut of this hierarchy
Distance Function
• Cosine measure (similarity measure)
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Text Clustering 9
Text Clustering
Agglomerative and Hierarchical Clustering
1. Form initial clusters consisting of a singleton object, and compute
the distance between each pair of clusters.
2. Merge the two clusters having minimum distance.
3. Calculate the distance between the new cluster and all other clusters.
4. If there is only one cluster containing all objects:
Stop, otherwise go to step 2.
Representation of a Cluster C
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Text Clustering 10
Text Clustering
Experimental Comparison
[Steinbach, Karypis & Kumar 2000]
Clustering Quality
 Measured as entropy on a prelabeled test data set
 Using several text and web data sets
 Bisecting k-means outperforms k-means.
 Bisecting k-means outperforms agglomerative hierarchical
clustering.
Efficiency
 Bisecting k-means is much more efficient than agglomerative
hierarchical clustering.
 O(n) vs. O(n2)

Text Clustering 11
Text Clustering
Suffix Tree Clustering
[Zamir & Etzioni 1998]
Forming Clusters
Not by similar feature vectors
But by common terms
Strengths of Suffix Tree Clustering (STC)
Efficiency: runtime O(n) for n text documents
Overlapping clusters
Method
1. Identification of “ basic clusters“
2. Combination of basic clusters

Text Clustering 12
Text Clustering
Identification of Basic Clusters
• Basic Cluster: set of documents sharing one specific phrase
• Phrase: multi-word term
• Efficient identification of basic clusters using a suffix-tree
Insertion of (1) “ cat ate cheese“
cat ate cheese
cheese
ate cheese
1, 1 1, 2 1, 3
Insertion of (2) “ mouse ate cheese too“
cat ate cheese
cheese
ate cheese
too too
mouse ate cheese too
too
1, 1 1, 2
2, 2
1, 3
2, 3
2, 1
2, 4

Text Clustering 13
Text Clustering
Combination of Basic Clusters
• Basic clusters are highly overlapping
• Merge basic clusters having too much overlap
• Basic clusters graph: nodes represent basic clusters
Edge between A and B iff |A  B| / |A| > 0,5 and |A  B| / |B| > 0,5
• Composite cluster:
a component of the basic clusters graph
• Drawback of this approach:
Distant members of the same component need not be similar
No evaluation on standard test data

Text Clustering 14
Text Clustering
Example from the Grouper System

Text Clustering 15
Text Clustering
Frequent-Term-Based Clustering
[Beil, Ester & Xu 2002]
• Frequent term set:
description of a cluster
• Set of documents containing
all terms of the frequent term
set: cluster
• Clustering: subset of set of all
frequent term sets covering
the DB with a low mutual
overlap
{} {D1, . . ., D16}
{sun} {fun} {beach}
{D1, D2, D4, D5, {D1, D3, D4, D6, {D2, D7, D8,
D6, D8, D9, D10, D7, D8, D10, D11, D9, D10, D12,
D11, D13, D15} D14, D15, D16} D13,D14, D15}
{sun, fun} {sun, beach} {fun, surf} {beach, surf}
{D1, D4, D6, D8, {D2, D8, D9, . . . . . .
D10, D11, D15} D10, D11, D15}
{sun, fun, surf} {sun, beach, fun}
{D1, D6, D10, D11} {D8, D10, D11, D15}

Text Clustering 16
Text Clustering
Method
• Task: Efficient calculation of the overlap of a given cluster
(description) Fi with the union of the other cluster
(description)s
• F: set of all frequent term sets in D
• f j : the number of all frequent term sets supported by document Dj
• Standard overlap of a cluster Ci:
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Text Clustering 17
Text Clustering
Algorithm FTC
FTC(database D, float minsup)
SelectedTermSets:= {};
n:= |D|;
RemainingTermSets:= DetermineFrequentTermsets(D, minsup);
while |cov(SelectedTermSets)| ≠ n do
for each set in RemainingTermSets do
Calculate overlap for set;
BestCandidate:=element of Remaining TermSets with minimum overlap;
SelectedTermSets:= SelectedTermSets  {BestCandidate};
RemainingTermSets:= RemainingTermSets - {BestCandidate};
Remove all documents in cov(BestCandidate) from D and from the
coverage of all of the RemainingTermSets;
return SelectedTermSets;

Text Clustering 18
Text Clustering
Joint Cluster Analysis [Ester, Ge, Gao et al 2006]
Attribute data: intrinsic properties of entities
Relationship data: extrinsic properties of entities
Existing clustering algorithms use either attribute or relationship data
Often: attribute and relationship data somewhat related, but contain
complementary information
 Joint cluster analysis of attribute and relationship data
Edges = relationships
2D location = attributes
Informative
Graph

Text Clustering 19
Text Clustering
The Connected k-Center Problem
Given:
• Dataset D as informative graph
• k - # of clusters
Connectivity constraint:
Clusters need to be connected graph components
Objective function:
maximum (attribute) distance (radius) between nodes and cluster center
Task:
Find k centers (nodes) and a corresponding partitioning of D into k clusters
such that
• each clusters satisfies the connectivity constraint and
• the objective function is minimized

Text Clustering 20
Text Clustering
The Hardness of the Connected k-Center Problem
• Given k centers and a radius threshold r, finding the optimal assignment for the
remaining nodes is NP-hard
• Because of articulation nodes (e.g., a)
Assignment of a implies
the assignment of b
Assignment of a to C1
minimizes the radiusa
b
C1 C2

Text Clustering 21
Text Clustering
Example: CS Publications
1073: Kempe, Dobra, Gehrke: “Gossip-based computation of aggregate information”, FOCS'03
1483: Fagin, Kleinberg, Raghavan: “Query strategies for priced information”, STOC'02
True cluster labels based on conference

Text Clustering 22
Text Clustering
Application for Web Pages
• Attribute data
text content
• Relationship data
hyperlinks
• Connectivity constraint
clusters must be connected
• New challenges
clusters can be overlapping
not all web pages must belong to a cluster (noise)
. . .

The General Clustering Problem
A clustering task may include the following components
(Jain, Murty, and Flynn 1999):
• Problem representation, including feature extraction, selection, or both,
• Definition of proximity measure suitable to the domain,
• Actual clustering of objects,
• Data abstraction, and
• Evaluation.
Text Clustering 23

The Vector-Space Model
• Assume t distinct terms remain after preprocessing; call them index terms or
the vocabulary.
• These “orthogonal” terms form a vector space.
Dimension = t = |vocabulary|
• Each term, i, in a document or query, j, is given a real-valued weight, wij.
• Both documents and queries are expressed as t-dimensional vectors:
dj = (w1j, w2j, …, wtj)
• New document is assigned to the most likely category based on vector
similarity.
Text Clustering 24

Graphic Representation
Text Clustering 25
T3
T1
T2
D1 = 2T1+ 3T2 + 5T3
D2 = 3T1 + 7T2 + T3
Q = 0T1 + 0T2 + 2T3
7
32
5
Example:
D1 = 2T1 + 3T2 + 5T3
D2 = 3T1 + 7T2 + T3
Q = 0T1 + 0T2 + 2T3
• Is D1 or D2 more similar to Q?
• How to measure the degree of
similarity? Distance? Angle?
Projection?

Document Collection
• A collection of n documents can be represented in the vector space model by
a term-document matrix.
• An entry in the matrix corresponds to the “weight” of a term in the document;
zero means the term has no significance in the document or it simply doesn’t
exist in the document.
Text Clustering 26
T1 T2 …. Tt
D1 w11 w21 … wt1
D2 w12 w22 … wt2
: : : :
: : : :
Dn w1n w2n …
T1 T2 …. Tt
D1 w11 w21 … wt1
D2 w12 w22 … wt2
: : : :
: : : :
Dn w1n w2n … wtn

Term Weights: Term Frequency
• More frequent terms in a document are more important, i.e. more
indicative of the topic.
fij = frequency of term i in document j
• May want to normalize term frequency (tf) by dividing by the frequency
of the most common term in the document:
tfij = fij / maxi{fij}
Text Clustering 27

Term Weights: Inverse Document Frequency
• Terms that appear in many different documents are less indicative of
overall topic
df i = document frequency of term i
= number of documents containing term i
idfi = inverse document frequency of term i,
= log2 (N/ df i)
(N: total number of documents)
• An indication of a term’s discrimination power.
• Log used to dampen the effect relative to tf.
Text Clustering 28

TF-IDF Weighting
• A typical combined term importance indicator is tf-idf weighting:
wij = tfij idfi = tfij log2 (N/ dfi)
• A term occurring frequently in the document but rarely in the rest of the
collection is given high weight.
• Many other ways of determining term weights have been proposed.
• Experimentally, tf-idf has been found to work well.
Text Clustering 29

Computing TF-IDF -- An Example
• Given a document containing terms with given frequencies:
A(3), B(2), C(1)
• Assume collection contains 10,000 documents and
document frequencies of these terms are:
A(50), B(1300), C(250)
Then:
A: tf = 3/3; idf = log2(10000/50) = 7.6; tf-idf = 7.6
B: tf = 2/3; idf = log2 (10000/1300) = 2.9; tf-idf = 2.0
C: tf = 1/3; idf = log2 (10000/250) = 5.3; tf-idf = 1.8
Text Clustering 30

Query Vector
• Query vector is typically treated as a document and also tf-idf
weighted.
• Alternative is for the user to supply weights for the given query
terms.
Text Clustering 31

Similarity Measure
• A similarity measure is a function that computes the degree of similarity
between two vectors.
• Using a similarity measure between the query and each document:
– It is possible to rank the retrieved documents in the order of presumed
relevance.
– It is possible to enforce a certain threshold so that the size of the
retrieved set can be controlled.
Text Clustering 32

Similarity Measure - Inner Product
• Similarity between vectors for the document di and query q can be computed
as the vector inner product (a.k.a. dot product):
sim(dj,q) = dj•q =
Where,
wij is the weight of term i in document j and
wiq is the weight of term i in the query
• For binary vectors, the inner product is the number of matched query terms in
the document (size of intersection).
• For weighted term vectors, it is the sum of the products of the weights of the
matched terms.
Text Clustering 33
iq
t
i
ij ww1

Properties of Inner Product
Text Clustering 34
• The inner product is unbounded.
• Favors long documents with a large number of unique terms.
• Measures how many terms matched but not how many terms are not
matched.

Text Clustering 3535
Inner Product -- Examples
Binary:
• D = 1, 1, 1, 0, 1, 1, 0
• Q = 1, 0 , 1, 0, 0, 1, 1
sim(D, Q) = 3
Size of vector = size of vocabulary = 7
0 means corresponding term not found in
document or query
Weighted:
D1 = 2T1 + 3T2 + 5T3 D2 = 3T1 + 7T2 + 1T3
Q = 0T1 + 0T2 + 2T3
sim(D1 , Q) = 2*0 + 3*0 + 5*2 = 10
sim(D2 , Q) = 3*0 + 7*0 + 1*2 = 2

Cosine Similarity Measure
Text Clustering 36
• Cosine similarity measures the cosine of the angle
between two vectors.
• Inner product normalized by the vector lengths.
D1 = 2T1 + 3T2 + 5T3 CosSim(D1 , Q) = 10 / (4+9+25)(0+0+4) = 0.81
D2 = 3T1 + 7T2 + 1T3 CosSim(D2 , Q) = 2 / (9+49+1)(0+0+4) = 0.13
Q = 0T1 + 0T2 + 2T3
2
t3
t1
t2
D1
D2
Q
1
D1 is 6 times better than D2 using cosine similarity but only 5 times better using
inner product.
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Naïve Implementation
• Convert all documents in collection D to tf-idf weighted vectors, dj, for
keyword vocabulary V.
• Convert query to a tf-idf-weighted vector q.
• For each dj in D do
Compute score sj = cosSim(dj, q)
• Sort documents by decreasing score.
• Present top ranked documents to the user.
Time complexity: O(|V|·|D|) Bad for large V & D !
|V| = 10,000; |D| = 100,000; |V|·|D| = 1,000,000,000
Text Clustering 37

Comments on Vector Space Models
• Simple, mathematically based approach.
• Considers both local (tf) and global (idf) word occurrence frequencies.
• Provides partial matching and ranked results.
• Tends to work quite well in practice despite obvious weaknesses.
• Allows efficient implementation for large document collections.
Text Clustering 38

Problems with Vector Space Model
• Missing semantic information (e.g. word sense).
• Missing syntactic information (e.g. phrase structure, word order, proximity
information).
• Assumption of term independence (e.g. ignores synonomy).
• Lacks the control of a Boolean model (e.g., requiring a term to appear in a
document).
– Given a two-term query “A B”, may prefer a document containing A
frequently but not B, over a document that contains both A and B, but
both less frequently.
Text Clustering 39

Clustering Algorithms
Problem variant
• A flat (or partitional) clustering produces a single partition of a set of
objects into disjoint groups.
• A hierarchical clustering results in a nested series of partitions.
Distance-based Clustering Algorithms
• Agglomerative and Hierarchical Clustering Algorithms
• Distance-based Partitioning Algorithms (k-Means)
• A Hybrid Approach: The Scatter-Gather Method (Buckshot,
Fractionization)
Text Clustering 40

Hard/Soft Clustering
Hard Clustering
• Every object may belong to exactly one cluster.
Soft Clustering
• The membership is fuzzy - Objects may belong to several clusters with a
fractional degree of membership in each.
Text Clustering 41

The most commonly used algorithms are
 The K-means (hard, flat, shuffling),
 The EM-based mixture resolving (soft, flat, probabilistic), and
 The HAC (hierarchical, agglomerative).
Text Clustering 42

Document hierarchical clustering
• Bottom-up, agglomerative
• Top-down, divisive
Document partitioning (flat clustering)
• K-means
• Probabilistic clustering using the Naïve Bayes or Gaussian mixture
model, etc.
Document clustering based on graph model
Text Clustering 43

K-Means Algorithm
Partitions a collection of vectors {x1, x2, . . . xn} into a set of clusters {C1, C2, . . . Ck}.
The algorithm needs k cluster seeds for initialization. The algorithm proceeds as
follows:
Initialization:
• k seeds, either given or selected randomly, form the core of k clusters.
• Every other vector is assigned to the cluster of the closest seed.
Iteration:
• The centroids Mi of the current clusters are computed:
• Each vector is reassigned to the cluster with the closest centroid.
Stopping condition:
• At convergence – when no more changes occur.
• The K-means algorithm maximizes the clustering quality function Q:
Text Clustering 44
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K-Means
Works when we know k, the number of clusters
Idea:
• Randomly pick k points as the “centroids” of the k clusters
• Loop:
–  points, add to cluster with nearest centroid
– Recompute the cluster centroids
– Repeat loop (until no change)
Text Clustering 45
Iterative improvement of the objective function:
Sum of the squared distance from each point to the centroid of its cluster.
Finding the global optimum is NP-hard.
The k-means algorithm is guaranteed to converge a local optimum.

K-means Example
For simplicity, 1-dimension objects and k=2.
• Numerical difference is used as the distance
Objects: 1, 2, 5, 6,7
K-means:
Randomly select 5 and 6 as centroids;
=> Two clusters {1,2,5} and {6,7}; meanC1=8/3, meanC2=6.5
=> {1,2}, {5,6,7}; meanC1=1.5, meanC2=6
=> no change.
Aggregate dissimilarity
– (sum of squares of distance each point of each cluster from its cluster
center--(intra-cluster distance)
= 0.52+ 0.52+ 12+ 02+12 = 2.5
Text Clustering 46
|1-1.5|2

K Means Example (K=2)
Pick seeds
Reassign clusters
Compute centroids
x
x
Reasssign clusters
x
x xx Compute centroids
Reassign clusters
Converged!

Time Complexity
• Assume computing distance between two instances is O(m) where m is the
dimensionality of the vectors.
• Reassigning clusters: O(kn) distance computations, or O(knm).
• Computing centroids: Each instance vector gets added once to some
centroid: O(nm).
• Assume these two steps are each done once for I iterations: O(Iknm).
• Linear in all relevant factors, assuming a fixed number of iterations, more
efficient than O(n2) HAC.
Text Clustering 48

Problems with K-means
Need to know k in advance
• Could try out several k?
– Cluster tightness increases with increasing K.
– Look for a kink in the tightness vs. K curve
Tends to go to local minima that are sensitive to the starting centroids
• Try out multiple starting points
Disjoint and exhaustive
• Doesn’t have a notion of “outliers”
– Outlier problem can be handled by K-medoid or neighborhood-based
algorithms
Assumes clusters are spherical in vector space
• Sensitive to coordinate changes, weighting etc.
Text Clustering 49

EM-based Probabilistic Clustering Algorithm
Mixture-resolving Algorithms - Underlying Assumption:
• The objects to be clustered are drawn from k distributions, and
• The goal is to identify the parameters of each that would allow the
calculation of the probability P(Ci | x) of the given object’s belonging to
the cluster Ci
The Expectation Maximization (EM)
• A general purpose framework for estimating the parameters of distribution
in the presence of hidden variables in observable data.
• Probabilistic method for soft clustering.
• Direct method that assumes k clusters:{c1, c2,… ck} and Soft version of k-
means.
• For text, typically assume a naïve-Bayes category model.
– Parameters  = {P(ci), P(wj | ci): i{1,…k}, j {1,…,|V|}}
Text Clustering 50

EM Algorithm
Initialization:
The initial parameters of k distributions are selected either randomly or
externally.
Iteration:
E-Step: Compute the P(Ci |x) for all objects x by using the current
parameters of the distributions. Re-label all objects according to the
computed probabilities.
M-Step: Re-estimate the parameters of the distributions to maximize the
likelihood of the objects’ assuming their current labeling.
Stopping condition:
At convergence - when the change in log-likelihood after each iteration
becomes small.
Text Clustering 51

EM
Unlabeled Examples
+ 
+ 
+ 
+ 
+
Assign random probabilistic labels to unlabeled data
Initialize:

EM
Prob.
Learner
+ 
+ 
+ 
+ 
+
Give soft-labeled training data to a probabilistic learner
Initialize:

EM
Prob.
Learner
Prob.
Classifier
+ 
+ 
+ 
+ 
+
Produce a probabilistic classifier
Initialize:

EM
Prob.
Learner
Prob.
Classifier
Relabel unlabled data using the trained classifier
+ 
+ 
+ 
+ 
+
E Step:

EM
Prob.
Learner
+ 
+ 
+ 
+ 
+
Prob.
Classifier
Continue EM iterations until probabilistic labels on
unlabeled data converge.
Retrain classifier on relabeled data
M step:

Hierarchical Agglomerative Clustering (HAC)
• Assumes a similarity function for determining the similarity of two
instances.
• Starts with all instances in a separate cluster and then repeatedly joins the
two clusters that are most similar until there is only one cluster.
• The history of merging forms a binary tree or hierarchy.
HAC Algorithm
Initialization:
Every object is put into a separate cluster.
Iteration:
Find the pair of most similar clusters and merge them.
Stopping condition:
When everything is merged into a single cluster.
Text Clustering 57

Cluster Similarity
Assume a similarity function that determines the similarity of two instances:
sim(x,y).
• Cosine similarity of document vectors.
How to compute similarity of two clusters each possibly containing multiple
instances?
• Single Link: Similarity of two most similar members.
• Complete Link: Similarity of two least similar members.
• Group Average: Average similarity between members.
Text Clustering 58

Clustering functions - Merging criteria
Text Clustering 59
Extending the distance measure from samples to sets of samples
Similarity of 2 most similar
members
Similarity of 2 least similar
members
Average similarity between
members

Single Link Agglomerative Clustering
Use maximum similarity of pairs:
Can result in “straggly” (long and thin) clusters due to chaining effect.
• Appropriate in some domains, such as clustering islands.
Text Clustering 60
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ji
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Single Link Example

Complete Link Agglomerative Clustering
Use minimum similarity of pairs:
Makes more “tight,” spherical clusters that are typically preferable.
Text Clustering 62
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ji

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Complete Link Example
Text Clustering 63

Computational Complexity
• In the first iteration, all HAC methods need to compute similarity of all
pairs of n individual instances which is O(n2).
• In each of the subsequent n2 merging iterations, it must compute the
distance between the most recently created cluster and all other existing
clusters.
• In order to maintain an overall O(n2) performance, computing similarity to
each other cluster must be done in constant time.
Text Clustering 64

Computing Cluster Similarity
After merging ci and cj, the similarity of the resulting cluster to any
other cluster, ck, can be computed by:
• Single Link:
• Complete Link:
Text Clustering 65
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)),(),,(min()),(( kjkikji ccsimccsimcccsim 

Group Average Agglomerative Clustering
• Use average similarity across all pairs within the merged cluster to
measure the similarity of two clusters.
• Compromise between single and complete link.
• Averaged across all ordered pairs in the merged cluster instead of
unordered pairs between the two clusters (to encourage tighter final
clusters).
Text Clustering 66
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ji jiccx xyccyjiji
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Other Clustering Algorithms
Minimal Spanning Tree (MST)
• Single-link clusters are sub-graphs of the MST, which are also the
connected components
• Complete-link clusters are the maximal complete sub-graphs
The Nearest Neighbor Clustering
• Assigns each object to the cluster of its nearest labeled neighbor object
provided the similarity to that neighbor is sufficiently high.
• The process continues until all objects are labeled.
The Buckshot Algorithm
• Combines HAC and K-Means clustering.
• First randomly take a sample of instances of size n
• Run group-average HAC on this sample, which takes only O(n) time.
• Use the results of HAC as initial seeds for K-means.
• Overall algorithm is O(n) and avoids problems of bad seed selection.
Text Clustering 67

Word and Phrase-based Clustering
• Text documents from high-dimensional domain, important clusters of words
may be found and utilized for finding clusters of documents.
Example:
In a corpus containing d terms and n documents, view a term-
document matrix as an n × d matrix, in which the (i, j)th entry is the
frequency of the jth term in the ith document.
• The problem of clustering rows in this matrix is clustering documents,
clustering columns in this matrix is clustering words.
• The most general technique for simultaneous word and document clustering
is referred to as co-clustering.
• The problem of word clustering is related to dimensionality reduction,
whereas document clustering is related to traditional clustering
• Probabilistic framework which determines word clusters and document
clusters simultaneously is referred to as topic modeling
Text Clustering 68

Word and Phrase-based Clustering
Approaches
• Clustering with Frequent Word Patterns
• Leveraging Word Clusters for Document Clusters
• Co-clustering Words and Documents
• Clustering with Frequent Phrases
Text Clustering 69

Clustering with Frequent Word Patterns
• A frequent item set in the context of text data is referred to as a frequent
term set.
• Frequent term sets is defined on the basis of the overlaps between the
supporting documents of the different frequent term sets.
Idea:
Cluster the low dimensional frequent term sets as cluster candidates.
Flat Clustering:
The documents covered by the selected frequent term are removed from the
database, and the overlap in the next iteration is computed with respect to
the remaining documents.
Hierarchical Clustering:
Each level of the clustering is applied to a set of term sets containing a fixed
number k of terms.
Text Clustering 70

Leveraging Word Clusters for Document Clusters
A two phase clustering procedure, to perform document clustering:
First Phase
• Determine word-clusters from the documents in such a way that most of
mutual information between words and documents is preserved when
representing the documents in terms of word clusters rather than words.
Second Phase
• Use the condensed representation of the documents in terms of word-
clusters in order to perform the final document clustering.
• Specifically, replace the word occurrences in documents with word-cluster
occurrences in order to perform the document clustering.
Advantage of two-phase procedure
• Significant reduction in the noise in the representation.
Text Clustering 71

Co-clustering Words and Documents
• Co-clustering is defined as a pair of maps from rows to row-cluster indices
and columns to column-cluster indices.
• These maps are determined simultaneously by the algorithm in order to
optimize the corresponding cluster representations
• Matrix Factorization Approach
 Discovers word clusters and document clusters simultaneously.
 Transform knowledge from word space to document space in the
context of document clustering techniques.
• Co-clustering is a form of local feature selection, in which the features
selected are specific to each cluster
• Methods for document co-clustering
 Co-clustering with graph partitioning
 Information-Theoretic Co-clustering
Text Clustering 72

Clustering with Frequent Phrases
Phrase cluster
• A phrase cluster is a phrase that is shared by at least two documents, and
the group of documents that contain the phrase.
• Each document is treated as a string of words, rather than characters.
• It uses an indexing method in order to organize the phrases in the
document collection, and then uses this organization to create the clusters.
Steps to create the clusters
Step 1:
• perform the cleaning of the strings representing the documents.
• A light stemming algorithm is used by deleting word prefixes and suffixes
and reducing plural to singular.
• Sentence boundaries are marked and non-word tokens are stripped.
Text Clustering 73

Clustering with Frequent Phrases
Step 2:
• Identification of base clusters.
• These are defined by the frequent phases in the collection which are
represented in the form of a suffix tree.
Step 3:
• An important characteristic of the base clusters created by the suffix tree is
that they do not define a strict partitioning and have overlaps with one
another
• The algorithm merges the clusters based on the similarity of their
underlying document sets.
Let P and Q be the document sets corresponding to two clusters. The base
similarity BS(P,Q) is defined as follows:
Text Clustering 74








 5.0
|}||,max{|
||
),(
QP
QP
QPBS

Benefits of Word Clustering
Useful Semantic word clustering
• Automatically generates a “Thesaurus”
Higher classification accuracy
Smaller classification models
• size reductions as dramatic as 50000  50
Text Clustering 75

Semi-supervised clustering: problem definition
Input:
• A set of unlabeled objects, each described by a set of attributes (numeric
and/or categorical)
• A small amount of domain knowledge
Output:
• A partitioning of the objects into k clusters (possibly with some discarded
as outliers)
Objective:
• Maximum intra-cluster similarity
• Minimum inter-cluster similarity
• High consistency between the partitioning and the domain knowledge
Text Clustering 76

Why semi-supervised clustering?
Why not clustering?
• The clusters produced may not be the ones required.
• Sometimes there are multiple possible groupings.
Why not classification?
• Sometimes there are insufficient labeled data.
Potential applications
• Bioinformatics (gene and protein clustering)
• Document hierarchy construction
• News/email categorization
• Image categorization
Text Clustering 77

Semi-Supervised Clustering
Domain knowledge
• Partial label information is given
• Apply some constraints (must-links and cannot-links)
Approaches
• Search-based Semi-Supervised Clustering
– Alter the clustering algorithm using the constraints
• Similarity-based Semi-Supervised Clustering
– Alter the similarity measure based on the constraints
• Combination of both
Text Clustering 78

Search-Based Semi-Supervised Clustering
Alter the clustering algorithm that searches for a good partitioning by:
• Modifying the objective function to give a reward for obeying labels on
the supervised data [Demeriz:ANNIE99].
• Enforcing constraints (must-link, cannot-link) on the labeled data during
clustering [Wagstaff:ICML00, Wagstaff:ICML01].
• Use the labeled data to initialize clusters in an iterative refinement
algorithm (k-Means,) [Basu:ICML02].
Text Clustering 79

Similarity-based semi-supervised clustering
• Alter the similarity measure based on the constraints
• Two types of constraints: Must-links and Cannot-links
• Clustering algorithm: Hierarchical clustering
Text Clustering 80

Transfer Learning
Text Clustering 81
It is motivated by human learning. People can often transfer knowledge
learnt previously to novel situations
 Chess  Checkers
 Mathematics  Computer Science
 Table Tennis  Tennis
TL - Solve the fundamental problem of different distributions between the
training and testing data.
Transfer Learning (TL):
The ability of a system to recognize and apply knowledge and skills
learned in previous tasks to novel tasks (in new domains)

Traditional ML vs. TL
Text Clustering 82
Traditional ML in
multiple domains
Transfer of learning
across domains
trainingitems
testitems
trainingitems
testitems
Humans can learn in many domains. Humans can also transfer from one
domain to other domains.

Traditional ML vs. TL
Text Clustering 83
Learning Process of
Traditional ML
Learning Process of
Transfer Learning
training items
Learning System Learning System
training items
Learning System
Learning SystemKnowledge

Why Transfer Learning?
• In some domains, labeled data are in short supply.
• In some domains, the calibration effort is very expensive.
• In some domains, the learning process is time consuming
Text Clustering 84
Transfer learning techniques may help!
 How to extract knowledge learnt from related domains to help learning
in a target domain with a few labeled data?
 How to extract knowledge learnt from related domains to speed up
learning in a target domain?

Settings of Transfer Learning
Text Clustering 85
Transfer learning settings
Labeled data in
a source domain
Labeled data in
a target
domain
Tasks
Inductive Transfer Learning
× √ Classification
Regression
…√ √
Transductive Transfer Learning √ ×
Classification
Regression
…
Unsupervised Transfer Learning × ×
Clustering
…

Approaches to Transfer Learning
Text Clustering 86
Transfer learning approaches Description
Instance-transfer
To re-weight some labeled data in a source
domain for use in the target domain
Feature-representation-transfer
Find a “good” feature representation that reduces
difference between a source and a target domain
or minimizes error of models
Model-transfer
Discover shared parameters or priors of models
between a source domain and a target domain
Relational-knowledge-transfer
Build mapping of relational knowledge between a
source domain and a target domain.

Approaches to Transfer Learning
Text Clustering 87
Inductive
Transfer Learning
Transductive
Transfer Learning
Unsupervised
Transfer Learning
Instance-transfer √ √
Feature-representation-
transfer
√ √ √
Model-transfer √
Relational-knowledge-
transfer
√

Unsupervised Transfer Learning
Feature-representation-transfer Approaches
Self-taught Clustering (STC)
[Dai et al. ICML-08]
Text Clustering 88
Input: A lot of unlabeled data in a source domain and a few unlabeled data in
a target domain.
Goal: Clustering the target domain data.
Assumption: The source domain and target domain data share some common
features, which can help clustering in the target domain.
Main Idea: To extend the information theoretic co-clustering algorithm
[Dhillon et al. KDD-03] for transfer learning.

Text clustering

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